comes completely constricted off as an elongated tube. His 

 conclusion as to the endodermal origin of the peribranchial 

 rudiment cannot, however, be unhesitatingly accepted, since 

 his figures do not satisfactorily establish the correctness 

 of his description, while the supposition that he has not 

 followed the development with sufficient care, is very strong, 

 Salensky (27: page 527) calls attention to the fact that the 

 little circle of epithelial cells which Pizon marks with the 

 letters Per in Fig. 7, Pi.T., "wohl auch einen Ouerschnitt 

 der unteren Wand des Kleimendarmes darstellen kann", and that 



naf a.ta.72 t>/~o t-e n /Aa.fi'/'cs 



it is the same structure as the pericardii™ figured in later 

 stages . 



The first appearance of the pericardial rudiment 

 observed by Hjort (8) in Botryllus , was a small clump of 

 cells lying against the ventral wall of the inner vesicle in 

 the posterior part of the bud to the right of the middle 

 line. As to the derivation of these cells, Hjort was unable 

 to say whether they were mesodermal cells or cells which had 

 wandered out from the endoderm, but he distinctly states that 

 an evagination of the inner vesicle does not occur at this 

 point. 



In the buds of Dlst a plia Salensky (27) observed a 

 similar collection of cells lying against the lower wall of 

 the branchial sac and surrounded by mesodermal cells. He 

 maintains that there is no ground for attributing an endo- 

 dermal origin to the rudiment, which is from the beginning 

 sharply marked off from the v/all of the branchial sac, and 



• 57- 



